1. Field of the Invention
The present invention relates to a CD-DVD compatible optical pickup using a twin laser diode emitting light of two different wavelengths and an optical recording and/or reproducing apparatus using the same, and more particularly, to a CD-DVD compatible optical pickup in which both on-axis light and off-axis light form a circular light spot on an optical recording medium and an optical recording and/or reproducing apparatus using the same.
2. Description of the Related Art
Generally, optical pickups are used in compact disc players (CDPs), CD-random access memory (ROM) drives, digital versatile disc players (DVDPs), DVD-ROM drives, etc. and perform data recording and/or reproduction on recording media in a non-contact manner.
An optical pickup used for the DVD-family recording media, such as a DVD or a DVD-ROM, should be compatible with that used for the CD-family recording media, such as a CD, a CD-R, a CD-RW, a CD-I, or a CD-G, to perform data recording and/or reproduction on two kinds of the recording media.
However, a standard thickness of a DVD is different from that of a CD due to an allowable tolerance of tilt of a disc and numerical aperture (NA) of an objective lens, etc. That is, the thickness between an incident surface and an information recording surface in a conventional CD-family recording media is 1.2 mm, whereas a thickness in a DVD is 0.6 mm. Thus, if an optical pickup for the DVD is used for the conventional CD-family recording media, spherical aberration is generated due to a thickness difference between the conventional CD-family recording media and the DVD. As such, sufficient optical intensity required for recording a data signal cannot be obtained, and a reproduction signal is deteriorated due to the spherical aberration.
Further, light sources for DVD and conventional CD family recording media reproduction have different standard wavelengths. That is, the wavelength of the light source for conventional CD-family recording media reproduction is about 780 nm, whereas the wavelength of the light source for DVD reproduction is about 650 nm.
Since a conventional CDP cannot reproduce data recorded on the DVD due to the above-described differences, a separate optical pickup for a DVD is required. Here, the separate optical pickup for the DVD must be compatible with that for the conventional CD-family recording media.
Considering the above-described problems, a CD-DVD compatible optical pickup as shown in FIG. 1 includes an optical output module 10 in which first and second light sources 11 and 13 are integrally formed, an objective lens 7 focusing light emitted from the first and second light sources 11 and 13 onto an optical recording medium 1, a beam splitter 5 converting a traveling path of incident light, and a photodetector 9 which receives light reflected from the optical recording medium 1 and then passed through the beam splitter 5.
The CD-DVD compatible optical pickup further includes a grating 3, a collimating lens 6, and an astigmatism lens 8.
The grating 3 is installed on an optical path between the optical output module 10 and the beam splitter 5 and diffracts and transmits the incident light. The collimating lens 6, disposed on an optical path between the beam splitter 5 and the objective lens 7, converges the incident light, and focuses light directed toward the objective lens 7 into parallel light. The astigmatism lens 8 is disposed on an optical path between the beam splitter 5 and the photodetector 9, and is inclined in a direction opposite to a direction in which the beam splitter 5 is inclined. The astigmatism lens 8 suppresses coma aberration and generates astigmatism in order to detect an error signal using an astigmatic method.
The objective lens 7 is mounted in a moving member (not shown) on an actuator 20. The actuator 20 drives the objective lens 7 in a tracking direction (direction along a X-axis in FIG. 1) of the optical recording medium 1 and a focusing direction (direction along a Y-axis in FIG. 1) based on a tracking error signal and a focusing error signal detected by the photodetector 9.
Referring to FIGS. 2 and 3, the optical output module 10 further includes a monitor photodetector 17 for monitoring an optical output of the first and second light sources 11 and 13 which emit light of two different wavelengths.
The first light source 11, emitting a first light beam 11a having the wavelength of about 650 nm, is employed in a case where a thin optical disc 1a (refer to FIG. 4), a thickness between an incident surface and a recording surface is relatively thin, is used as the optical recording medium 1. The second light source 13, emitting a second light beam 13a having the wavelength of about 780 nm, is employed in a case where a thick optical disc 1b (refer to FIG. 4), a thickness between an incident surface and a recording surface is relatively thick, is used as the optical recording medium 1. The second light source 13 is used in, for example, an optical pickup for a CD utilizing a relative long wavelength.
The first and second light sources 11 and 13 are a twin laser diode and are installed on a mount 15 in a single chip shape. Further, the first and second light sources 11 and 13 are separately installed by grooves 12 so as to irradiate light of two different wavelengths. In the optical output module 10 having the above-described structure, the distance between an emitting point of the first light source 11 and an emitting point of the second light source 13 is optically about 110 μm. Thus, in a case where the optical elements are disposed such that a center of the first light beam 11a emitted from the first light source 11 travels along the optical axis of the light as indicated by the solid lines in FIG. 1, the second light beam 13a emitted from the second light source 13 deviates from the optical axis of the light as indicated by the dotted lines in FIG. 1.
FIG. 4 shows that the first and second light beams 11a and 13a emitted from the first and second light sources 11 and 13 pass through the objective lens 7 and then are focused onto the optical recording medium 1. As shown in FIG. 4, since the first light beam 11a traveling along the optical axis has an optical profile symmetric with respect to the optical axis, the first light beam 11a is incident on the incident surface of the optical recording medium 1 at substantially a right angle. On the other hand, since the second light 13a travels asymmetrically with respect to the optical axis, the second light beam 13a is incident on the incident surface of the optical recording medium 1 at a predetermined angle other than substantially a right angle.
Thus, as shown in FIG. 5A, the first light beam 11a preferably forms a circular light spot SP1 on a thin optical disc 1a. However, as shown in FIG. 5B, the second light beam 13a forms a light spot SP2 of a distorted shape on a thick optical disc 1b. As a result, optical aberration and jitter increase in the light spot SP2 formed by the second light beam 13a so that the signal reproduction performance of an optical recording and/or reproducing apparatus using the optical pickup is reduced.